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Register and download the demo mode resources CrOS component during
online demo mode setup. This happens before online enrollment starts.

Also updates offline-enrolled Demo Mode to always use the offline
resources image instead of attempting to download the CrOS component.

Bug: 869455
Change-Id: I6a947eab916f68bba4730a9ab611136be7fe1bc1
Reviewed-on: https://chromium-review.googlesource.com/1198018
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589355}